EJB 3.0 core specfication, section 13.1.2:
The Enterprise JavaBeans architecture supports flat transactions. A flat transaction cannot have any child (nested) transactions.
Note: The decision not to support nested transactions allows vendors of existing transaction processing and database management systems to incorporate support for Enterprise Java-Beans. If these vendors provide support for nested transactions in the future, Enterprise Java-Beans may be enhanced to take advantage of nested transactions.
